1. Field of the Invention
The present invention relates to a method of fabricating an ohmic metal electrode for use in nitride compound semiconductor devices and, more particularly, to a method of fabricating a p-type ohmic metal electrode that uses Ru and RuO.sub.x as the cover layer. The present invention also relates to a method of fabricating an n-type ohmic metal layer that uses Ru as the diffusion barrier layer. Here, a nitride compound semiconductor refers to Al.sub.x In.sub.y Ga.sub.1-(y+x) N, where 0 .ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, and x+y.ltoreq.1.
2. Description of the Prior Art
Fabrication of ohmic contacts is one of the techniques that are essential in fabricating opto-electronic devices such as nitride-base blue/green and white light emitting diodes (LED's), ultraviolet (UV) detectors, and laser diodes (LD's) and electronic devices such as high electron mobility transistors (HEMT's), metal-insulator-semiconductor field effect transistors (MISFET's), and heterostructure bipolar transistors (HBT's).
Conventionally, p-type ohmic metal electrodes for use in nitride-base optical devices and electronic devices employed Au, such as in Ni/Au, as the cover layer. However, various pollutants such as oxygen in the air penetrate through the Au cover layer to reach the underlying contact layer, thereby forming an oxide pollutant layer at the junction of the contact layer and the nitride semiconductor. Thus, the structural and thermal stability of the p-type ohmic metal electrode experiences degradation during the heating process, and the electrical, thermal, and optical characteristics are deteriorated.
In the case of n-type ohmic metal electrodes, an ohmic metal electrode with a Ti contact layer has been conventionally used. However, unnecessary interaction occurred between the contact layer and the nitride semiconductor during the heating process, and thus the thermal characteristics was not good despite good electrical characteristics. In order to overcome this disadvantage, diffusion barrier layers employing Ni or Pt between Ti/Al and Au, such as in Ti/Al/Ni/Au or Ti/Al/Pt/Au, were used. However, the results were not satisfactory.
In particular, in case of Au-base p-type ohmic metal electrodes, the optical output characteristics were significantly degraded because the photons generated in the optical devices were absorbed into the Au.
One of the most important techniques that need to be developed in fabricating laser diodes having a nitride compound semiconductor substrate is to develop an ohmic metal electrode having low resistance, high light transmittance, and thermal stability. However, there has been neither prior art nor any published research regarding Au-free p-type ohmic metal electrodes.
S. J. Pearton, et al. of the U.S.A. published their research regarding fabricating an ohmic contact layer on p-type GaN by using W and WSi in the American Institute of Physics in 1998 (Applied Physics Letters 73, 942, 1998). The object of the research was to develop an ohmic contact layer having thermal stability and good electrical characteristics. However, according to the publication, thermal stability was good, but specific contact resistance determinative of electrical characteristics was 10.sup.-2 .OMEGA.cm.sup.2, which is worse than 10.sup.-4 .OMEGA.cm.sup.2, the minimum acceptable value for optical devices.
Murakami, et al. published their research regarding Ta/Ti contact system in the American Institute of Physics in 1999 (Applied Physics Letters 74, 275, 1999). According to the publication, the specific contact resistance was 10.sup.-5 .OMEGA.cm.sup.2, which was very good, but there was a problem of instability at room temperature.
There were also reports on various researches regarding ohmic metal electrodes having other types of structures, but there has been no research resulting in superior characteristics sufficient for use in actual devices.